1. Field of the Invention
The present invention relates to a device and to a method for controlling cooking areas of a cooking unit with glass-ceramic cooking surfaces.
2. Related Art
The temperatures of the cooking areas of cooking units with glass-ceramic cooking surfaces are limited using mechanical temperature-protecting controllers. DE-A 40 22 846 and EP-A 0 823 620 disclose sensors which are mounted directly on the glass-ceramic cooking surface. They measure the temperature by via the change of the electrical resistance of a conductor track or of the glass-ceramic base material of the cooking surface in question. Monitoring the temperature directly on the glass-ceramic cooking surface is advantageous since it avoids any overheating or overloading of the glass-ceramic cooking surface and the cooking utensil, which may be standing thereon. Direct glass-ceramic temperature sensing hence provides additional safety. AT-A 238 331 discloses a cooking unit provided with a cooking area with a switching device for the power supply of its heater. The switching device enables the power supply to the heating device when the cooking utensil is put on it, and disables the energy supply when the cooking utensil is removed.
DE-A 35 33 997 and DE-A 33 27 622 disclose cooking utensil detection systems with optical detectors. Cooking utensil detection systems with inductive detectors are described in DE-A 37 11 589 and DE-A 37 33 108. Inductive proximity switches are based on the principle of damping a tuned circuit due to eddy-current losses in metals which are located in the magnetic leakage field of a multiturn sensor coil. Cooking utensil detection systems with an inductive sensor in the form of a coil are described in EP-A 0 442 275 and EP-A 0 469 189. Capacitive sensors for cooking utensil detection are disclosed by WO-A 90/107851 and EP-A 0 429 120.
DE-A 197 00 735 describes a device for inductive detection of cooking pans. The sensors consist of one single-turn coil for the transmitter and one for the receiver, which are arranged concentrically as circles in the cooking area. In this case, the coils may be formed by conductor tracks fitted on a. support plate, for example a glass-ceramic cooking plate.
The following disadvantages are encountered with the existing systems:
Two different systems, such as coils for inductive cooking utensil detection and mechanical temperature-protecting controllers, with different components are currently used for the combination of cooking utensil detection and temperature monitoring.
DE-A 196 46 826 discloses a device and method for temperature measurement and cooking utensil detection on cooking tops. This patent describes a device for measuring the temperature of a glass-ceramic cooking top and/or for detecting a cooking utensil, which is being heated on the cooking top. In this case, at least one sensor is fitted to the cooking top, in particular immediately below the cooker top. The sensor is designed as a conductor track, wherein a temperature-dependent electrical resistance is measurable between its two outer terminal ends. Two measurement terminals are branched off from the conductor-track, the two branch points delimiting a central lead section (which is remote from the outer terminal ends) of the conductor track for temperature measurement.
In a combination of cooking utensil detection according to DE-A 197 00 753 and temperature limitation according to DE-A 40 22 846 as well as EP-A 0 823 620), at least six terminals are required for each cooking area. Furthermore, the conductor tracks for the supply leads of the temperature detector need to be laid between the conductor tracks of the supply leads for the cooking utensil detection. Therefore, the supply leads for cooking utensil detection in the cold region enclose such a large area that cooking utensils placed in the cold region can cause the cooking area to be switched on, which produces a considerable safety risk.
It is an object of the present invention to provide a novel device for controlling cooking areas with glass-ceramic cooking surfaces, which involves a smaller number of components, simpler connection and safer operation, as well as an economical and safe method for controlling cooking areas with glass-ceramic cooking surfaces.
The object of the invention is achieved by a device for a controlling cooking area with a glass-ceramic cooking surface and at least one conductor track structure. The at least one conductor track structure comprises a combined device consisting of at least one inductive cooking utensil detection device and at least one temperature measurement device. The at least one temperature measurement device includes means for measuring a temperature of the glass-ceramic material or glass-ceramic surface located between at least two conductor tracks of the combined device. The at least two conductor tracks include a conductor track for temperature measurement arranged in or extending through the edge region and another conductor track arranged in or extending through the central region of the cooking area.
By virtue of this combination, the same detectors are used for cooking utensil detection and for temperature measurement, so that a simplified system involving a smaller number of components, simpler connection and safer operation is obtained according to the invention. Furthermore, it is therefore possible to arrange the terminals of the inner conductor structure for the cooking utensil detection directly next to each other, so that the enclosed area is minimized and safe operation is made possible by avoiding malfunction due to switching on the cooking area when cooking pans are put in the cold region.
The result achieved according to the invention is that the structures relevant to the cooking utensil detection are kept in the edge region of the cooking area. The maximum signal amplitude, which occurs when the inner circuit is covered by a cooking utensil that has been put on the cooking unit, can thus be tuned with the cooking utensil size matched to the cooking area size. In addition, it is possible to sense the inner region of the cooking area, which is important for temperature limitation, especially in the event of the wrong pan. Another advantage of the arrangement is that the temperature detector senses the edge region. Stressful situations, such as offset cooking utensils, which produce high temperatures in the edge region, are detected (see the draft standard DIN VDE 0700 Part 6, Annex 8). The relative sensitivity of the temperature measurement between the edge and the inner region can be adjusted by the respective conductor lengths in the individual regions and by the conductor track spacing in these regions.
The cooking surface according to the invention preferably contains a conductor track coating for measuring, monitoring and displaying the temperature which is used for automatically distributing the power, for switching off in the event of a boil dry condition and for limiting the temperature. The cooking surface also-contains a conductor track coating for automatically detecting a cooking utensil, the shape of cooking utensils and the size of cooking utensils. Surprisingly, and unexpectedly, the arrangement according to the invention is more economic for the user, increases safety during use and saves considerable energy.
In a preferred embodiment according to the invention the conductor track for the temperature measurement extending through the central region is designed as an open-ended electrode.
In another preferred embodiment according to the invention the conductor track for the temperature measurement in the central region is designed as a looped electrode.
In another particularly preferred embodiment of the invention, the leads for the inner conductor track circuit are arranged immediately next to each other. This arrangement according to the invention advantageously overcomes problems and prevents a reduction in safety from occurring when a cooking utensil is not positioned centrally on the cooking area.
A multi-circuit-heating element is provided according to the invention, wherein the innermost heating circuit is designed according to the invention. The device according to the invention is suitable for use in multi-circuit-heating elements.
An economical method for controlling cooking areas with glass-ceramic cooking surfaces having a conductor track structure according to the invention comprises providing a combined device or conductor track structure for both cooking utensil detection and temperature measurement and then performing the cooking utensil detection and temperature measurement with it.
In a preferred embodiment of the method according to the invention the temperature is measured by evaluating the temperature-dependent conductor track resistance. Good results are achieved by evaluating the temperature-dependent conductor track resistance.
In other preferred embodiments the temperature is measured by evaluating the temperature-dependent resistance of the glass-ceramic material between at least two conductor tracks. Very good results are achieved by this latter evaluation.
In further preferred embodiments of the method according to the invention the temperature is measured in the edge region and in the central region of the cooking area. The entire heating area is very advantageously sensed with this arrangement.
The temperature is measured in the central region by means of an open-ended electrode or by means of a looped electrode in preferred embodiments of the invention.
In various other preferred embodiments the signals of the cooking utensil detection device and of the temperature measurement device are separated by different frequencies for the cooking utensil detection and for the temperature measurement. In a preferred embodiment of the method according to the invention the respective signals of the cooking utensil detection device and of the temperature measurement device are separated by time-lagged interrogation.
The relative sensitivities for the temperature measurement in the outer region and the inner region are set by selecting the lengths and spacing of the conductors in preferred embodiments of the method.
The signals of the cooking utensil detection device and of the temperature measurement device, which are picked up using the same sensors, are separated by using and outputting AC voltages with different frequencies. For instance, the cooking utensil detection device is operated at from 10 MHz to 17 MHz, preferably from 11 MHz to 15 MHz, and particularly preferably from 12.5 MHz to 13.5 MHz. The temperature measurement device is operated at 150 Hz to 600 Hz, preferably from 200 Hz to 400 Hz, and particularly preferably from 250 Hz to 350 Hz.
Sequential interrogation of the two measured variables is also possible.
In the embodiment indicated, the conductor track region for the temperature measurement at the cooking area center does not contribute to the signal from the cooking utensil detection device, since the electrode extending from the outer circuit is not designed as a loop, and therefore no current flows through it. It is hence not inductively active.
In the case of two-circuit cooking areas, the additional conductor track structure at the cooking area center is derived only from the cooking utensil detection of the inner heating circuit. Using this structure, the critical inner region of a cooking pan is sensed in the case of both small and large diameters. The outer region of the large heating circuit may in this case be sensed by a separate temperature measurement by the outer cooking utensil detection structure, as is described for carrying out power redistribution in DE-A 40 22 846.35. The temperature measurement may be used for applications which are sufficiently described in the literature, such as temperature control, power redistribution, residual heat display or automated cooking. The applications are described in DE-A 21 39 828, DE-A 40 22 846 and DE-A 37 44 372, to which reference is explicitly made here.
The arrangement according to the invention has the following advantages. The same detectors are used for the cooking utensil detection and temperature measurement. The number of sensors used is halved in comparison with conventional systems. Mechanical temperature-protecting controllers are hence unnecessary. The number of terminals for inductive cooking utensil detection using conductor tracks is not increased by the additional function of temperature measurement. The supply leads to the conductor tracks for the cooking utensil detection can be laid close together and malfunctions due to putting pans in the cold region are prevented.